* ClosingFuture}s. Use {@link #whenAllSucceed(ClosingFuture, ClosingFuture)} to start this
   * combination.
   *
   * @param <V1> the type returned by the first future
   * @param <V2> the type returned by the second future
   */
  public static final class Combiner2<V1 extends @Nullable Object, V2 extends @Nullable Object>
      extends Combiner {

    /**